1. Field of the Invention
This invention relates generally to an image pick-up tube and, more particularly, is directed to an apparatus for controlling the deflection of an electron beam in an image pick-up tube.
2. Description of the Prior Art
FIG. 1 is a diagram showing an example of a prior art image pick-up tube of electromagnetic focusing and electrostatic deflection type. In FIG. 1, reference numeral 1 designates a glass bulb, 2 a face plate, 3 a target plate or screen (photoelectric conversion screen), 4 an indium ring for cool-sealing, and 5 a metal ring. Reference numeral 6 designates a metal electrode which is inserted through the face plate 2 so as to contact the target screen 3 for drawing out a signal therefrom.
Reference letters K, G.sub.1 and G.sub.2 designate a cathode, a first grid and a second grid, respectively, which constitute an electron gun. In this case, reference letter LA designates a beam limiting aperture which limits the divergence angle of an electron beam Bm supplied to the target screen 3 from the electron gun.
Reference letter G.sub.3 designates a third grid electrode which forms a deflection electrode. This third electrode grid G.sub.3 is formed of a metal, such as chromium, which is deposited by vacuum evaporation or plated on the inner surface of the glass bulb 1 and then cut in a predetermined pattern by, for example, a laser beam. In this case, in order to produce a uniform deflection electric field, the third grid electrode G.sub.3 is formed as a so-called arrow pattern as shown in the developed view of FIG. 2. In FIG. 2, reference letters V+ and V- designate vertical deflection electrodes, respectively, to which saw-tooth wave voltages E.sub.V+ and E.sub.V- of the vertical period changing symmetrically around a predetermined voltage are applied. Further, in FIG. 2, reference letters H+ and H- respectively designate horizontal deflection electrodes to which saw-tooth wave voltages E.sub.H+ and E.sub.H- of the horizontal period changing symmetrically around a predetermined voltage are applied. Thus, the vertical and horizontal deflection scannings are carried out.
Turning back to FIG. 1, reference letter G.sub.4 designates a mesh-shaped electrode which is supported on a mesh holder 7. Reference numeral 8 designates a focusing coil and reference numeral 9 designates a stem pin.
In such image pick-up tube, most of the electron beam Bm emitted from the electron gun is absorbed by the target screen 3, the mesh electrode 4 and the like. However, there is a case where a part of the electron beam Bm returns to the electron gun, reflected thereon, accelerated again and then reaches to the target screen 3. When such electron beam is landed on a so-called over-scan area 3B (FIG. 3) of the target screen 3, a false signal caused by this secondary beam reflection is generated. Particularly when the potential at the over-scan area 3B is different from that at the effective image area 3A, the possibility that the secondary-reflected beam will land on the over-scan area becomes high. Furthermore, in such image pick-up tube, the effective image area 3A and the over-scan area 3B of the target screen 3 as shown in FIG. 3 are different in potential so that a so-called framing is caused thereby. In other words, the potential is disturbed around the periphery of the effective image area 3A and a shadow just like a frame one occurs.
Therefore, in order to avoid the generation of the false signal caused by the secondary beam reflection, the following methods are proposed: first a transparent electrode is formed only on the necessary portion of the target screen 3; second the area of the mesh-shaped electrode G.sub.4 is decreased, or a so-called mesh angle is made small so that the beam can be prevented from being landed on the target screen 3 except the effective image area; and third the electron gun is formed so as to avoid the generation of the false signal caused by the secondary beam reflection. However, in accordance with the first method, the area of the transparent electrode can not be reduced so much because of problems, such as accuracy of the centering and so on; in accordance with the second method, the area of the mesh-shaped electrode can not be reduced so much because if the area of the mesh-shaped electrode is reduced, the electric field is disturbed and, as a result, mis-landing of the electron beam will be caused; and in accordance with the third method, the electron gun of satisfactory construction has not yet been produced and further, such previously proposed methods can not avoid the generation of the false signal caused by the secondary beam reflection completely.
As disclosed in U.S. Pat. No. 4,439,713 having a common assignee herewith, in order to avoid the occurrence of framing, it has been previously proposed that the target screen 3 be scanned by the electron beam Bm at the over-scan area 3B formed near the periphery of the effective image area 3A, or that the so-called over-scan be carried out. According to this over-scan, the charge accumulated in the over-scan area 3B near the periphery of the effective image area 3A is discharged and thereby a potential difference between the peripheral over-scan area 3B and the effective image area 3A can be removed so that it is possible to avoid the framing caused by the potential difference between the over-scan area 3B and the effective image area 3A. Further it is proposed that the velocity of the electron beam Bm is made high in this over-scan portion and hence the electron beam Bm can over-scan the long distance within a short duration of time. More particularly, the saw-tooth wave voltages E.sub.V+ and E.sub.V- for vertical deflection are formed to have waveforms as shown in FIGS. 4A and 4B, each of which has a steep inclination in an over-scan portion los. In FIGS. 4A and 4B, T.sub.BLK represents the vertical blanking period. Saw-tooth wave voltages E.sub.H+ and E.sub.H- for horizontal deflection are formed to have over-scan waveforms similar to the saw-tooth wave voltages E.sub. V+ and E.sub.V- though not shown.
It is to be noted that even though the reflected electron beam exists, if it is not landed on the over-scan area 3B of the target screen 3, the false signal caused by the secondary beam reflection is not generated. It is possible to reduce the possibility that the secondary beam reflection will be landed on the over-scan area 3B by making the potential of the over-scan area 3B equal to that of the effective image area 3A. Thus, the over-scan method is effective for reducing the generation of the false signal caused by the secondary beam reflection.
However, according to the above-described over-scan method, as shown by the scanning loci thereof in FIG. 3, the scanning of the over-scan area 3B, for example, in the vertical direction, gets very rough, and the area which cannot be scanned is considerable with the result that the potential thereof is not lowered sufficiently so that the avoidance of the generation of the false signal by the over-scan method cannot be fully achieved. In other words, there are spaces or gaps between the successive scans by the focused beam on the over-scan area, so that the potential is not adequately reduced at such spaces or gaps.